Zero pressure live roller conveyors began appearing in the marketplace in the early 1970's as flat belt-driven live roller conveyors comprised of multiple drive zones that would either engage to transport cartons or disengage to stop transporting cartons. The state of engagement within a zone normally was dependent on whether there was a carton present within a downstream zone. A sensor roller protruded slightly above the top plane of the conveyor roller bed and would be depressed by the weight of a carton. When the sensor roller was depressed, the drive in the adjacent upstream zone would disengage and enter a neutral state. Once cartons were conveyed off the sensor roller, the sensor roller would return to its original position above the plane of the conveyor roller bed and the conveyor in the adjacent zone upstream would reengage to transport cartons.
It soon became apparent that the sensor roller and associated air valve were cycling with the passage of each carton. The valves were exhausting air and then refilling with each cycle. While this cycling was necessary when cartons were accumulating, it wasted vast amounts of compressed air when cartons were simply transporting. Many ideas began to surface in order to conserve the compressed air. Some ideas consisted of incorporating a small orifice in the pneumatic conduit to slow down the exhausting of air from the associated actuators whenever the sensor roller was depressed. This scheme also slowed down the filling of actuators, but it did manage to sharply reduce the volume of compressed air used during transportation.
U.S. Pat. No. 3,768,630 to Inwood et al. is an example of another scheme. Inwood monitors the discharge of the conveyor to see if the cartons are to be transported or accumulated. If product is to be transported, the sensor valves are serially bypassed so that any air lost during the cycling is merely from the small amount of volume within the hoses connecting the valve to the actuators. However, if one of the sensor valves would not exhaust properly, conveyor drive from that point upstream would continue whether the sensor roller was depressed or not. This would create crushing pressure on the cartons and performance failure.
Since the time of Inwood, conveyor speeds and the number of cartons being conveyed has increased significantly. The associated noise, fatigue and impact forces on the sensor roller and valve began to take its toll with high failure rates and maintenance attention required. U.S. Pat. No. 5,316,130 to Heit et al. used retractable sensor rollers that solved both the air volume and roller/valve cycling problems by retracting the sensor rollers below the conveying surface whenever the conveyor was transporting. When the conveyor was accumulating, the sensor rollers were allowed to pop up. U.S. Pat. No. 5,358,097 to Bakkila et a. and U.S. Pat. No. 5,429,225 to Schiesser et al. were variations on this concept.
However, the prior art still has shortcomings that reduce the lifespan and performance of accumulating conveyors. One shortcoming of the prior art was the need to attach hoses to moving parts. With the light forces involved with the sensor roller and valve, any variation in hose length or position could keep the valve or the crank arm from moving to its intended position. Thus, if the amount of hose available for flexing is not just right or if the orientation of hose fittings is not just right, the assembly becomes useless. This is particularly troublesome when maintenance personnel make repairs in the field.
The present invention provides an advantage in performance over the prior art. The referenced patents all describe a scheme that has the sensor roller retract as soon as the actuators within the zone are pressurized. That is, when the first zone is pressurized, the first sensor roller is retracted and the first sensor valve begins to pressurize the second zone actuators very quickly. This sequence repeats all the way along the length of the conveyor with any cartons on board transporting with very little space between individual cartons and with no regard for whether cartons are present in the zone or not. If the production rate of the downstream conveyor is slower than the speed of the accumulation conveyor, there will be a build up of pressure as the accumulation conveyor tries to push more cartons than can be transported away. The referenced patents anticipate this condition and try to delay the retracting by using small orifice restrictions in the air lines. Depending on speed, desired discharge rate and product weight, the size of orifice must be varied to suit each individual product line.